Firstly, you will need to review the guidance on residence at RDR3 (RDR3 Statutory Residence Test
) and take the residency tests.
Tax on foreign income UK residence and tax
If you are considered not resident in the UK, you only declare the self employment income from the date you arrived in the UK to the end of the tax year.
If you are not resident in the UK, then you need to declare this on supplemenary page SA109 (Residence, remittance basis etc (Self Assessment SA109)